As compared with the traditional gravitational casting systems, bottom casting in accordance with low pressure casting processes offers considerable advantages. For one, the metal melt does not come into contact with the oxygen of the air on its way from the pressure furnace receptacle up to the foundry mold. The rising pipe of the pressure furnace, which fits against the pouring gate opening below or on the lower part of the foundry mold, terminates close to the bottom where, in most cases the inductor is disposed so that, advantageously, the extraction of the melt takes place at the hottest location. Furthermore, no impurities can be carried forward from the top of the bath into the foundry mold. In low pressure casting, there exist no problems with variable heights of fall and deflectors of the jet, which are present in gravitational casting, for these can be omitted. It is advantageous, above all, that considerable reduction of the lost circulating material is possible primarily because the material in the vertically extending pouring gate empties back into the pressure furnace at the conclusion of the casting process.
German AS 25 58 449 (corresponding to U.S. Pat. No. 4,008,749) shows a low pressure casting process for the filling of vertically divided foundry molds. In foundry molds with a vertical separating plane, it is possible to mold in casting bays or runners extending slantingly downwardly which can also serve as feeders. Such slanting casting bays are not possible when using horizontally divided foundry molds, which are by far used most often. According to the above-mentioned German AS, several casting bays with small cross sections connect the pouring channel with the hollow spaces of the mold. The pressure on the bath level in the pressure furnace must be maintained after the filling process until the casting runners freeze up, i.e., until the melt in the casting runs solidifies. Only then is the melt in the pouring channel permitted to drop back into the pressure furnace. As a result, extended waiting times are necessary, leading to a reduced degree of effectiveness and reduced production rate. Moreover, there is the danger, which cannot be disregarded, that pressure breaks might develop so that the melt will run out.